Transponder antenna ground plane - Help!

[Bastoune]

While finalizing the installation of a TRIG TT21 transponder, I am running into less that consistent information regarding the ground plane spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod antenna inside the fiberglass fuselage.

The TRIG TT21 Manual has this to say about the dimension of the ground plane:
"[...] As the ground plane is made smaller, the actual dimensions of the ground plane become more critical, and small multiples of the wavelength should be avoided, as should circles. Rectangles or squares are much less likely to create a critical dimension that resonates with the transmissions. The smallest practical ground plane is a SQUARE around 120mm per side; as the size increases the performance may actually get worse, but will be better by the time the ground plane is 700mm on each side."

From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly states that "The smallest recommended ground plane would be 12 inches in diameter (305mm)."

Now, when I look at all of the Schleicher TNs, I see that they call out in most case a 130mm ROUND aluminum disk to be used in conjunction with the RAMI antenna mentioned above.

So what gives? Round or Square? Short of a very large plane, what dimension to use for the ground plane? The fact that the information available does not seem to converge on a given dimension or shape for a ground plane tells me that this is probably more than one "OK" answer.

Thanks,
B

[Darryl Ramm]

Does your glider manufacturer have a TN or similar for a transponder
install in this or a similar glider ? if so just follow those directions.
Ground planes are usually better off on the larger side and round. If
making this up a fuselage internal antenna install yourself you *might* be
better off with a dipole antenna that does not need a ground plane.

The Schleicher TN ground plane diameters are often limited by internal
clearance, however again you should follow the glider manufacturers
directions if they exist. There are reasons that an external antenna
install may be preferred to an internal one.

It is fantastic that you are installing a transponder, but I get nervous
with questions like this, if you are having to ask these questions are
there other things as well you are overlooking or misunderstanding? it is a
great idea if you can to have an experienced glider A&P or knowledgable
person check on your plans well before you start the install.

Darryl

[Dan Marotta]

When I installed my TT22, I made an aluminum square, 120mm on the side, and
curved it to fit the inside of my fuselage aft of the landing gear. ATC has
no problem seeing me.


"Darryl Ramm" <darry...@gmail.com> wrote in message
news:257681905416772990.8750...@reader80.eternal-september.org...

[mpa...@gmail.com]

Antenna design is determined by physics; the size is determined by the transmit frequency. Other than where the antenna goes, the particular aircraft has nothing to do with antenna design or size.

The minimum for a ground-plane antenna is a ground plane of 4 wires (or rods), that are 1/4 wavelength long, and that extend radially from the base of the center vertical element. This is the type of ground plane antenna that you see at an airport ground station. (bear in mind that an aircraft transponder antenna is upside down from a base-station antenna) The transponder transmit frequency of 1090 mhz will give you 1/4 wavelength ground plane rods (or wires) of 68.8 mm length.

The next best thing is a multitude of 68.8mm long radial wires until you have so many that you ultimately end up with a metal disc with a radius of 68.8 mm. A round metal circle requires some degree of accuracy.

The third best thing is a square where the distance from the center to one side is less than 1/4 wavelength and the distance from the center to a corner is more than 1/4 wavelength. That is why one of the examples is a square with a side of 120mm. The square is nice because it is very forgiving of inaccuracy.

The best is an extremely large (as a ratio of the ground plane to the height of the vertical element) ground plane. Considering how short the vertical element is, 700mm is large enough. Due to the large size, this is not suitable in a glider.

bigger is not necessarily better. As you get bigger than the minimum, there are some sizes that are good and some sizes that are bad. As the TRIG TT21 Manual states, anything in between the minimum and very large "may" make performance worse. And that is why you have gotten so many answers.

My suggestion is that you go to to the RAMI site and clicking on "contact tech support". The next best suggestion is that you either use the 68.8mm radius circle or the 120mm per side square. Make sure you round the corners for safety.

[Dan]

If you have a fiberglass fuselage I'd suggest using the L-2 antenna.
It doesn't require a ground plane and is easy to install
Dan

[2G]

On Monday, March 17, 2014 7:41:38 AM UTC-7, Bastoune wrote:

One way to fabricate a ground plane is to use copper foil tape that uses conductive adhesive. Just overlap it in strips on the inside of the fuselage around the antenna.

Tom

[drguya...@gmail.com]

Really...why do you have to make this so complicated?
Get an L-2. Providing you have a pure fiberglass fuselage just slap it on the inside of the fuse and be happy.
I have an LS-8. The L-2 is on the side of the fuse behind the wheel. Air traffic control radar reads me just fine no matter what my orientation to the radar. Commercial aircraft that are above me read me no matter what our relative orientations.

[2G]

Need I say more?

[Bastoune]

Thanks for the info. An A&P & experienced avionic installation centre will be doing the install and xpdr test so it will be their call at the end. I am just trying to understand why there are varying ground plane recommendations between the trig manual, RAMI antenna mfr and what Scheichler typically recommends. Mpa-Thank you for the explanation. I know understand a bit better the trade-offs in the ground plane sizes. I saw the L-2 antenna. Seems nice.

[Bill T]

Don't let them cut your fiberglass, use the internal dipole described earlier glued/velcroed to an internal verticle brace. We did that for an Ls4, Grob 103 and Grob 104.

BT

[Bob Kuykendall]

On Monday, March 24, 2014 8:37:57 PM UTC-7, Bill T wrote:
> Don't let them cut your fiberglass...

That's all well and good if it is indeed fiberglass. I can't remember if the original poster said what their fuselage is made of. But if it's carbon fiber, an internal antenna will not work because of the opacity of carbon to radio waves.

With carbon fuselages, you have to either put the antenna somewhere that isn't carbon (sometimes the fin and/or rudder are made with radio-transparent fiberglass or aramid fibers), or you must install an external antenna.

With my gliders, the fuselage and fin is made with carbon, but we put the com antenna in the aramid fiber rudder. We haven't installed any transponder antennas yet, but when we do we'll probably just install an external blade antenna and call it good.

Thanks, Bob K.

[ifee...@hotmail.com]

Since you mentioned the Schleicher TN and the Schleicher TN's list the RAMI AV-22 as a suitable antenna I assume you're installing the transponder in a Schleicher. RAVI's installation instructions on their website give no guidance as to sizing the ground plane as they only refer to externally mounting the antenna on a metal airframe, but as you found their engineer is quoted on the Cumulus website as recommending 30cm minimum diameter... I would ask them and refer them to the aircraft manufacturer's TN spec of 13cm and see if they think it's alright.

Just for good measure the AC43-13 says that a transponder ground plane should be 12 inches diameter minimum.

Isn't t it nice to get contradictory information from the authorities? Confusion is fun!

I'm not sure whose instructions are considered the final authority under the FAA. Perhaps it's the aircraft manufacturer's as they give your A&P and avionics shop the only instructions specifically about your glider to use to authorize the installation. It seems unlikely that Schleicher's instructions would result in unacceptable performance though given how many gliders flying in EASA airspace must have had transponders installed in accordance with them by now.

The L2 antenna does seem convenient but it sounds like the original poster requires a TSO'd antenna. The L2 isn't so far as I know.